This invention relates to methods and apparatus for detecting the point at which a conduit is stuck in a borehole.
When a conduit, such as a drill string, is stuck in a borehole, a conventional method for determining the depth of the stuck point is to apply torsional and tensional deformations to the conduit from the surface and determine the depth to which the deformations are transmitted. A suitable apparatus is lowered into the conduit at the end of a cable and positioned at successive depths for detecting the deformations in the conduit.
A conventional stuck point detection apparatus, described for example in U.S. Pat. No. 3,686,943, comprises a body member having a top part and a bottom part mounted movably in relation to each other. Top and bottom anchoring elements mounted respectively over these top and bottom parts immobilize each of the parts of the body member in two longitudinally spaced zones of the conduit. Electric motors driven via the cable are used for moving the anchoring elements away from and toward the body member. Detection means mounted between the parts of the body member measure the relative movements between these parts when the conduit is deformed elastically by stresses applied from the surface.
An improved anchoring method, described in U.S. Pat. No. 4,104,911, consists first in anchoring the top part of the body member, and then, after releasing the tension on the cable, anchoring the bottom part. The advantage is that the moving parts of the body member are not placed in compression for the measurement. During this measurement, however, it is necessary for the two parts of the body member to be in a longitudinally contracted position to be able to move away from each other when tensions are applied to the drill pipes. For this purpose, the apparatus generally includes an elastic element or spring which applies a biasing force to the parts of the body member to load them toward each other. The drawback of such prior-art apparatus resides in the opposition of the return force to relative longitudinal extensions between the two parts of the body member during detection.
A first object of the invention therefore is to minimize such a longitudinal biasing force during the measurement.
For the detection of movements between the parts of the body member, it is advantageous to use a transducer such as described in U.S. patent application Ser. No. 258,218, filed Apr. 27, 1981 now U.S. Pat. No. 4,351,186 (assigned to the assignee of the present invention), which delivers two separate output signals, one independent of the rotations and the other independent of the longitudinal movements between the parts of the body member. This particular apparatus includes a resetting system which follows a complicated sequence of operations to place the detection means in an initial angular position before the measurement. In other known apparatus, the angular resetting system includes a spring or other elastic means, the return torque of which opposes rotation between the two parts of the body member during detection.
A second object of the invention is to provide an automatic angular resetting system not entailing a return torque during measurement.
Finally, customary resetting systems are generally driven by drive elements which are specific to them and controlled externally.
Another object of the invention is therefore to provide an apparatus whose resetting takes place automatically after anchoring the bottom part of the body member.